The interface doesn't have to be simple. What it should be is conforming to established UI patterns and conventions. Blender used to be incredibly unintuitive even to people who had never used any other 3D modeler before.
On the other hand: there is no reason whatsoever something as trivial as drawing a circle has to be as complicated as launching a rocket in GIMP. It certainly doesn't lack the technological scaffolding, and UX-wise people would already be ecstatic if they just cloned how 30-year-old version of Microsoft Paint did it. So why doesn't it have a Circle Tool yet, despite the massive amount of requests for it? The only possible explanation left seems to be an active disdain towards basic non-technical people: the UI is hard to use because they want it to be hard to use.
Another example is Gimp. People like to bag on it for having a terrible interface, but when they say Photoshop is so much better I have to wonder what magical version they are using. For me the differences between the two are marginal, but that may be because I learned how to use Gimp first and have to hunt around Photoshop's interface more.